FUTURE PLANS AND TECHNICAL DEVELOPMENTS
FOR THE SIBERIAN SOLAR RADIO TELESCOPE

	G.Ya.Smolkov, A.T.Altyntsev, V.V.Grechnev, B.B.Krissinel,
	S.V.Lesovoi, V.P.Maksimov, A.M.Uralov, and V.G.Zandanov
	Institute of Solar-Terrestrial Physics SB RAS, Irkutsk, 664033, Russia
	smolkov@iszf.irk.ru
	Abstract: How, for what purposes and in what conditions was the SSRT created? Short description and current modernization of the SSRT. New scientific objectives and technical requirements to the SSRT. Avenues for technical developments for the SSRT.

 The development of the large radio telescope project, designed for addressing the problems in the physics of the solar corona, was conceived at the beginning of the 1960s with the purpose of switching over from recording of active regions and flares using their integral emission to a systematic study of the atmospheric structure of active regions, the spatio-temporal processes of emergence of magnetic fluxes and their interaction, to identification of flare buildup signatures, and to the localization of energy storage and release regions. Incidental observations of solar eclipses, when high angular resolution at least in one coordinate was achieved, did not answer these questions. The project was aimed at achieving an angular resolution an order of magnitude higher than existing radioheliographs. On this basis, we hoped for a detailed study of the development of events in the corona at the background of the solar disk, for a possibility of improving the scientific framework of flare prediction, as well as expecting to come nearer to the study of the mechanism of development of flares and accompanying events. It must be remarked that neither experience nor adequate financial and technological means were available to us. For that reason, we had to develop the project phase by phase, submit it to official expert examination and carry out the prototype testing of design solutions until, in the 1970s, we were able to embark on the construction of the instrument that was named the Siberian Solar Radio Telescope (SSRT) [1, 2].

The initial instrumentation enabled us to record only the most powerful manifestations of solar activity: active regions, and flares. The lack of the necessary computer and receiver technologies dictated the particular technique of formation of the beam and the principle of operation of the SSRT. Progress was achieved by resolving the problems of phasing the 128-element equidistant antenna arrays of long electrical length (1.2x10⁴ wavelengths), synchronous tracking of the Sun throughout the daytime in climatic conditions of Siberia, recording the radio brightness distribution of the solar corona, generating solar radio images, and automating the operation and controlling all systems of the territorially distributed SSRT complex. The successful phasing of such multi-element antenna arrays added in creating, in 1992, the 17 GHz  radioheliograph at Nobeyama Observatory (Japan). The creation of the SSRT signified the advent in Russia of systematic radioheliography and a meaningful breakthrough in solar radio astronomy. In addition to the SSRT, the ISTP operates another two large astrophysical observatories (the high-altitude observatory in the Sayan mountains, and the observatory on Lake Baikal’s shore) for solar research in the optical range [3]. The infrared telescope is under development; it is designed for solar corona observation in the light of the line of He10830 A. Together with the SSRT, they constitute a large astrophysical complex in East Siberia. The ISTP enjoys one of the leading scientific schools of Russia in the field of  research on solar activity and solar-terrestrial connections; a wealth of experience on scientific instrument making has been accumulated.

"The SSRT is a crossed..."

	The SSRT is a crossed interferometer operating at 5.2 cm wavelength and comprising 256 antennas 2.5 m in diameter each, arranged in the form of 128 antennas at 4.9 m intervals along the north-south and east-west directions (Fig. 1). The SSRT’s field of view is on the order of 90 min of arc. Some redundancy of the number of antenna elements was admitted in order to achieve higher sensitivity, good quality of the spectrum of spatial frequencies, and to simplify the phasing problem and the baseline for a further development of the instrument. Sequential-continuous recording of the radio brightness distribution of the solar corona is carried out through parallel multichannel  reception of emission during the Sun’s transit through diffraction maxima (occupying the sky) using the frequency-dependence of the beam orientation (“scanning” in declination in the frequency band 112 MHz,  Fig. 2) and rotation of Earth (scanning in the hour angle). The frequency band that is required for collecting signals from the antennas via the branched parallel-cascaded waveguide system, was 120 MHz.  The initial signal frequency is converted twice. Parallel recording is carried out via 180 channels of 500 kHz width. All this provides solar radio images every several minutes (see examples in Figs. 3 and 4). Simultaneous records are taken of the responses of one-dimensional arrays of the instrument which are used to study fast processes during flares with a time resolution of 14 ms. Main SSRT parameters are presented in Table 1.

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 The process of mastering the SSRT has been accompanied by an expansion of the scientific interests and a change of the requirements to its characteristics. The advent of new technologies and instrumentation made it possible to embark on the modernization of the SSRT systems. The use of SHF amplifiers with a noise temperature of 50 degrees allowed the SSRT sensitivity to be improved considerably, enabling the recording and study of low-contrast features in the solar corona: coronal holes, filament, and bright coronal points. The receiving-recording system is being improved through the use of acoustooptic facilities. The methodology of improving the solar corona radio image quality is under development. It has been possible to achieve a spatial and temporal resolution of up to 15 sec of arc and up to 14 ms, respectively.
     The relatively high angular and, especially, temporal resolution of the SSRT, together with the adequate sensitivity, made it possible to considerably extend our research to almost all forms of solar activity: new models were proposed (and the old models refined) for active regions, flares, formation and acceleration of energetic particle fluxes, coronal mass ejections, and coronal holes. First and foremost the spatial-temporal characteristics of the origination and development of active regions were studied. A detailed study was made of the phenomenon of the sign reversal of the circular polarization of emission from the active region during its solar disk passage, a number of new flare buildup signatures were identified, an effective criterion of flare ‘protonicity’ from the character of emission polarization distribution of the active region was developed, we studied the properties and

suggested some likely generation mechanisms for subsecond impulses of microwave emission during flares and investigated the spatial structures of coronal holes associated with their atmospheric heating characteristics [4]. The research efforts are carried out in collaboration with observatories of Japan, China, Europe and the USA using data obtained at ground-based and orbital observatories in all ranges of solar emission.
     However, two factors: the obsolescence of the SSRT systems and the need to switch over to advanced diagnostics of events in the Sun’s atmosphere which call for substantial improvement of the speed for obtaining radio images, a simultaneous recording of the processes occurring under different physical conditions, or a three-dimensional picture of their development - led us in 1997 to the decision to create - on the basis of modernizing the SSRT - a new-generation instrument, the multiwave radioheliograph (MVRH) [5-7]. That decision is in full accord with the tendency established in the 1990s in radioheliography (introduction of a second frequency at NRH, development of OVRO, the proposal on the development and construction in the USA of FASR at ~0.3-30 GHz  and LOFAR at 15-150 MHz , the decimetric array in Brazil at 1.2-1.7, 2.7 and 5.0 GHz,  and Chinese Space Solar telescope (SST), the radioheliograph at 1-30 MHz  with high speed and resolution). The implementation of these projects involves overcoming very challenging technical problems and requires considerable funds. With their creations, scientists will have instruments at their disposal, which would meet - for many years ahead - all conceivable needs of solar physics and solar-terrestrial connections. They will revolutionize the space weather prediction problem.

 We are well aware of the expediency of creating the new instrument meeting all demands of the near future. Therefore, we fully support the ideas behind the creation of the above-mentioned radio telescopes. Considering the realistic financial conditions in Russia, the ISTP is working on two versions of SSRT conversion to MVRH which differ by the speed, the methods of frequency conversion, collection of signals from the antennas, and by the rate of obtaining radio images (in the modes of frequency scanning and aperture synthesis). They show that in Russia it is possible to create - with moderate expenses - a multiwave radioheliograph in the frequency range 2-10 GHz.  This instrument cannot compete with FASR as regards the number of frequencies and spatial resolution but will provide the necessary data for successful investigations in the field of highly important problems in solar physics, including the space weather prediction problem. The difference of the longitudinal location of the SSRT (MWRH in the future) and FASR will make it possible to carry out mutually complementary observations of the Sun on a 24-hour basis. The basic expenses incurred by the implementation of the ISTP project reduce virtually to reequipment of the SSRT by additional instrumentation; the antenna system, the receiving-recording complex, the automation system, all civil engineering part and infrastructure of the SSRT complex will be used in full measure. Alterations may apply to the method and, hence, the system of signal collection. Substantially smaller expenses will be required for the replacement of the obsolescent CAMAC-modules (by modern microprocessors), cables, as well as for the overhaul of the waveguide tunnel (water proofing, protection devices). MWRH will be accessible to all interested observatories.

                                      Scientific objectives of MWRH
     1. Magnetography of the corona - determination of the structure and strength of magnetic fields. The effectiveness of magnetic field measurements in the active region corona was demonstrated by RATAN and VLA observations, with limitations in spatial resolutions and the field of view, respectively. MWRH would provide measurements of the field at a number of levels in the chromosphere and corona (estimations of the magnetic field in the chromosphere from the Zeeman effect are made difficult by the structure of spectral lines). Angular resolution varies linearly with frequency. Therefore, even in the case of a discrete set of frequencies, MWRH can provide contours of field levels at the base of the corona by assuming the emission from the 3rd gyrolevel B = 120 f (GHz).  The coincidence of the SSRT and NRH observing intervals brings the number of frequencies to eight. In relatively weak fields, information about the magnitude of the magnetic field along the line of sight is furnished by the degree of polarization. Investigation of oscillatory processes at different frequencies (levels in the solar atmosphere) is also associated with this problem.

 2. Solar flares. Radio methods are no less sensitive to observations of high-energy electrons than X-ray methods. Through the use of several frequencies it will be possible to determine the spectral index of emitting electrons, and the picture of the polarization - the direction of magnetic fields. In many cases it is possible to observe weaker structural elements and understand the topology of flare loops. Determination of the magnitudes of magnetic fields in the flaring region - the most important parameter for estimating the energy release. Observation of the shortest-duration manifestations of solar activity associated with plasma mechanisms of generations. In the study of flares it is also planned to use one-dimensional observations with high temp[oral resolution.
     3. Coronal mass ejections. As demonstrated by experience of investigating the CME that occurred on September 4, 2000, radio mapping provides essentially new information about the origin of coronal mass ejections. The MWRH spectral range make sit possible to investigate the process of CME development from the lower corona to altitudes on the order of one solar radius (at the background of the solar disk and in the height range inaccessible to observations in the H-alpha line, SOHO coronographs, and to m-radioheliographs). Simultaneous observations of all Sun in microwave emission will make it possible to study the association of CMEs with other forms of solar activity: flares, shock waves, and filament eruption.

 4. Structures inside coronal holes, distribution of plasma parameters in holes with the height.
     5. Bright coronal points - local energy release in the corona. They are observed at the SSRT and NRH simultaneously. The brightness temperature ratio is indicative of the bremsstrahlung mechanism of emission. Nevertheless, two frequencies are insufficient for identifying the plasma parameters.
     6. Applications. Short-term prediction of geoeffective solar flares, base don a number of signatures (polarization structure depending on the structure of the photospheric magnetic field and on the active region location on the solar disk, evolution of the microwave emission flux, etc.). Recording of CMEs. Diagnostics of solar flare parameters. Alerts within the space weather program. Education of young scientists with the new instrument.
     The solution of the above-mentioned problems substantially alters and enhance the requirements for new solar radio telescopes.  The MWRH project provides for the attainment of a time resolution on the order of 1 s, and the sensitivity and dynamic range will be sufficient for recording all forms of solar activity, the angular resolution can be improved using remote antennas (such as the antenna 32 m in diameter that is already under construction near the SSRT for the Russian RSDB “Quasar” system).

 With the frequency scanning of the Sun preserved, it is intended to equip each antenna with a multiwave feeder and a system of synthesizers to covert the signal frequency at the new frequencies to the SSRT’s working frequency in order to  make use of the existing wideband system of signal collection, and to temporally separate the recording of data at different frequencies [5-7] (Fig. 5). Elements from this version are already being worked out by co-participants. However, this version does not provide radio images with the necessary temporal resolution (on the order of 1 s). Already at the present time the SSRT observations are impossible to use in cooperative programs such as HIGH CADENCE IMAGING CAMPAIGN: MEDOC. Therefore, design solutions are under development for switching over from many-frequency scanning to aperture synthesis. Preliminary results show that this approach is realistic at present, as there is no need to develop an expensive correlator, and with the present level of computer technology, its function can be performed by a sufficiently powerful processor. In this case it will be possible to map a full solar disk at intervals of about 1 s. Added expenses will be incurred by the use of fiber-optic communication lines via antennas, switching elements, and by the purchase of the processor.
     Consequently, the use of parallel aperture synthesis is more promising because to solve the above-mentioned problems requires obtaining solar radio images at different wave lengths with sufficiently high temporal resolution. It is obvious that this can be realized only through the use of parallel aperture synthesis.

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 To do this, it is necessary to equip the SSRT antenna elements with multiwave convertor modules (MCM). Each MCM will comprise the polarization switch, the low-noise amplifier, the frequency converter with the synthesizer as the heterodyne, the attenuator, the phase shifter, and the microprocessor. It is admissible to use six wavelengths in the frequency range from 2 GHz  to 10 GHz  (2340, 3100, 4340, 5730, 7815 and 0300 MHz).  MCM converts signals of all waves received by the antenna, to an intermediate frequency. Synthesizers at all antennas are synchronized by the phase-stable reference signal. Signals at the intermediate frequency are fed to the working room for a correlation processing.
     An important characteristic of MWRH would be the simultaneous obtaining of the spatial spectrum at different wavelengths. To accomplish this, it is possible to use the redundancy of the SSRT antenna system. Based on the objective of flare
and coronal mass ejection observation, the changes in spatial structures of our interest are occurring in the region of high spatial frequencies. To record them simultaneously, it is sufficient to fill the edges of the SSRT array with antennas operating at different frequencies (Fig. 6). For measuring coronal magnetic fields, the instrument will operate in the mode of sequential use of working wavelengths (100...500 ms for each of the six working wavelengths). For observing a quiet Sun, active regions, filaments, and coronal holes, use will be made of the redundancy of the SSRT antenna system to simultaneously obtaining spatial spectra at different wavelengths. Thus we expect to obtain a flexible instrument for performing different tasks simultaneously.

 It is intended to use the mode of sequential obtaining of images at different frequencies in order to study coronal magnetic fields, in addressing problems of predicting solar activity, and in investigating low-contrast structures such as coronal holes and filaments. Coronal mass ejections are commonly characterized by the angular size as large as several tens of min of arc. Therefore, it is more appropriate to investigate such phenomena in the mode of sequential obtaining full solar images at different frequencies. Observations of ejections in the mode of simultaneous obtaining a full solar image are appropriate only when accompanied by fast occurring processes. For investigating the dynamics of solar activity (flares and, possibly, coronal mass ejections), it is intended to use the mode of simultaneous obtaining of a full solar image at 5.7 GHz  frequency and high spatial harmonics of corresponding images at the other wavelengths. A tentative configuration of antennas for this mode is illustrated in Fig. 6. The field of view for frequencies above 5.7 GHz  in this case will be reduced to a few min of arc. This should not have a substantial influence on the observations of flares as a consequence of their small angular size. However, in observations of coronal mass ejections, such a limitation could have a serious influence.
     It should be noted that the question as to the selection of the antenna configuration for simultaneous obtaining of images at different frequencies is open. It seems most likely that it will be answered in the process of observations. For this it is important that the whole instrument has a sufficient flexibility. Such a flexibility is ensured by equipping each antenna with a controlled multiwave receiver and by the control of the process of calculating cross-correlation functions of signals received from antennas.

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 Key issues of the MWRH project: the number of antenna elements and the configuration of using them at the new frequencies, frequency-agile feeders, versions of collection of signals from antennas, heterodynes of frequency conversion 1 and 2, fiber-optic lines to supply the heterodyne signal to antennas, separation and digitizing data at all frequencies, data preprocessing and storage, SolarSoft, system analysis, and matching of all subsystems. Cooperation, exchange of experience or sharing of labor with relevant teams of the FASR project are possible and expedient in the process of working out these issues.
     State of affairs. There is well-established cooperation of manufacturers of the necessary additional equipment. Control systems and systems for controlling over antennas, phase shifters and synthesizers are under development using the state-of-the-art designs. New acoustooptic receivers are being produced, which permit, already in the year 2002, recording two-dimensional images every 2-3 minutes simultaneously with one-dimensional scans from both beams, with a resolution of 14 ms. Receivers, amplifiers and synthesizers for 6 frequencies are under development. The technology for building multiwave feeders is being worked out. Thus, with relatively low expenses (of about 15 million rubles), in a period of two-three years it is possible to obtain a qualitatively new instrument which outperforms the existing solar radio telescopes for the set of their characteristics: the two-dimensional mode of operation with high spatial resolution at six wavelengths, the possibility of observations throughout the daytime, the possibility of carrying out patrol of fast processes on the full disk with high temporal and angular resolution.

 Optimistic hopes for the project implementation plan:

Completion of working out the engineering issues      2002

Completion of prototype testing of design solutions   2003

Drawing out of the necessary documentation     2002-2003

Solving the problem of funding the project implementation       2002-2003

Project implementation                                        2003-2005

       SUMMARY 1
An important characteristic of MWRH would be the simultaneous obtaining of the spatial spectrum at different wavelengths. Obtaining radio images of the solar corona during the daytime by recording all spatial scales is possible through the implementation of adaptive multiwave approach. The essence of the method is as follows. All SSRT antennas will be equipped with multiwave modules. The adaptive multiwave mode make sit possible to realize a redundancy of the SSRT antenna array. For calibrations and for observations of slowly varying structures, it is intended to obtain solar radio images consecutively at different wavelengths. Specifically, either all SSRT antennas or one and the same optimum configuration will be used for each wavelength. Thus it is intended to obtain a quality image of all Sun at six frequencies. The only limitation will be the overlay of noon images at frequencies higher than the SSRT design frequency, 5.7 GHz.  This will be caused by the mismatch of the lowest spatial frequency to the angular size of the Sun for these frequencies. The use of all SSRT antennas in synthesizing the image make sit possible to obtain a highest-quality image but increases substantially (several times) the amount of information. Therefore, the selection of the particular configuration in this case will be influenced not only by the physical expediency but also by the overall performance of the system.

SUMMARY 2
1.     The relatively high angular and, especially, temporal resolution of the SSRT, together with the adequate sensitivity, made it possible to considerably extend our research to almost all forms of solar activity.

 2.     However, two factors: the obsolescence of the SSRT systems and the need to switch over to advanced diagnostics of events in the Sun’s atmosphere which call for substantial improvement of the speed for obtaining radio images, a simultaneous recording of the processes occurring under different physical conditions, or a three-dimensional picture of their development - led us in 1997 to the decision to create - on the basis of modernizing the SSRT - a new-generation instrument, the multiwave radioheliograph (MVRH).

 3.     We are well aware of the expediency of creating the new instrument meeting all demands of the near future. Therefore, we fully support the ideas behind the creation of the FASR and other new radio telescopes. Considering the realistic financial conditions in Russia, the ISTP is working on two versions of SSRT conversion to MVRH which differ by the speed, the methods of frequency conversion, collection of signals from the antennas, and by the rate of obtaining radio images (in the modes of frequency scanning and aperture synthesis). They show that in Russia it is possible to create - with moderate expenses - a multiwave radioheliograph in the frequency range 2-10 GHz.

4.     The difference of the longitudinal location of the SSRT (MWRH in the     future) and FASR will make it possible to carry out mutually complementary observations of the Sun on a 24-hour basis.

5.      Scientific objectives of MWRH: Magnetography of the corona, Solar flares, CME, Structures inside coronal holes, Bright coronal points, Applications.

6.     The solution of the above-mentioned problems substantially alters and enhance the requirements for new solar radio telescopes.  The MWRH project provides for the attainment of a time resolution on the order of 1 s, and the sensitivity and dynamic range will be sufficient for recording all forms of solar activity, the angular resolution can be improved using remote antennas (such as the antenna 32 m in diameter that is already under construction near the SSRT for the Russian RSDB “Quasar” system).

7.     Consequently, the use of parallel aperture synthesis is more promising because to solve the above-mentioned problems requires obtaining solar radio images at different wave lengths with sufficiently high temporal resolution. It is obvious that this can be realized only through the use of parallel aperture synthesis.

8.     Key issues of the MWRH project: the number of antenna elements and the configuration of using them at the new frequencies, frequency-agile feeders, versions of collection of signals from antennas, heterodynes of frequency conversion 1 and 2, fiber-optic lines to supply the heterodyne signal to antennas, separation and digitizing data at all frequencies, data preprocessing and storage, SolarSoft, system analysis, and matching of all subsystems. Cooperation, exchange of experience or sharing of labor with relevant teams of the FASR project are possible and expedient in the process of working out these issues.

9.     An important characteristic of MWRH would be the simultaneous obtaining of the spatial spectrum at different wavelengths. To accomplish this, it is possible to use the redundancy of the SSRT antenna system. We expect to obtain a flexible instrument for performing different tasks simultaneously.

10.    MWRH will be accessible to all interested observatories.

 We greatly appreciate the support of the work from the Siberian Branch of the Russian Academy of Sciences (SB RAS), the Ministry of Industry and Science of the RF (Unique facilities: 01-27), the RFBR (00-02-16456, 00-02-16819, 00-15-96710, 01-02-162900 and INTAS (IR-1088).
 
REFERENCES
1.     Smolkov G.Ya., Pistolkors A.A., Treskov T.A., Krissoinel B.B., Putilov V.A. and Potapov N.N.. The Siberian Solar Radio Telescope: parameters and principle of operation, objectives and results of first observations of active regions and flares. Astrophysics and Space Science, 119(1986), 1-4.
2.     Smolkov G.Ya., Treskov T.A., Krissinel B.B., Miller V.G., Grechnev V.V., and Konovalov S.N. The Siberian Solar Radio telescope. In: Regional
     Monitoring of the Atmosphere”, pt. 3. Unique Measuring Complexes.
     Novosibirsk: Nauka, Izd-vo SO RAN, 1998, Ch. 3, pp.85-149.
3.     Smolkov G.Ya., Stepanov V.E., Grigoryev V.M., Banin V.G. The East-Siberian complex of SibIZMIR solar observatories.  Astrophysics and Space Science 118 (1986), 21-30.

4.     G.Ya.Smolkov, A.T.Altyntsev, V.V.Grechnev, B.B.Krissinel, S.V.Lesovoi, V.P.Maksimov, A.M.Uralov and V.G.Zandanov. Some researches results from the SSRT. Poster for the Workshop “Solar Radiophysics with the FASR” (May 23-25’2002, Green Bank, West Virginia).
5.     Zandanov V., Altyntsev A. and Lesovoi S. Development of the Siberian Solar Radio Telescope.  Nobeyama Symposium on Solar Physics with Radio Observations (Oct.27-30’1998, Kiyosato). Abstract book. NRO/NAOJ, 1998, pp.86-87.
6.     V.G.Zandanov, G.Ya.Smolkov, A.T.Altyntsev, T.A.Treskov, B.B.Krissinel, S.V.Lesovoi, V.P.Maksimov. On development of  SSRT. Wayfang Symposium (July’1999, China).
7.     Smolkov G.Ya., Zandanov V.G., Altyntsev A.T. Towards the creation of a      new-generation radioheliograph. Proc. of SPIE, 2000, v. 4015, pp.197-203.

Thank You